Low-density microcellular carbon foams from sucrose by NaCl particle templating using glycerol as a plasticizing additive
Graphical abstract
Introduction
Carbon foams are new generation thermo-structural materials considered for aerospace applications due to their lightweight, tunable thermal conductivity, fire resistance and ablative property [1], [2]. They also possess hydrophobic nature, tunable electrical conductivity, high surface area, high electromagnetic interference shielding effectiveness and acoustic absorption property [3], [4], [5], [6], [7]. They find a variety of applications such as thermal management, fire-resistant structures, catalyst support, battery electrodes, EMI shielding material and oil spill remediation [8], [9], [10], [11], [12], [13]. The cellular and reticulated carbon foams are prepared by foaming and polyurethane foam templating methods, respectively, using various precursor materials of synthetic or natural origin [14], [15], [16], [17]. The fossil-based materials such as coal & pitch and renewable materials such as sucrose, tannin, and lignin are the natural precursors reported for the preparation of carbon foams [18], [19], [20], [21], [22], [23], [24]. Some of the synthetic polymeric materials used are phenol-formaldehyde resin, cyanate ester resin, polyimide, poly (arylacetelene) and polybenzoxazine [16], [25], [26], [27], [28]. These carbon foams have a relatively large cell size that limits their mechanical strength. The brittle carbon foams with large cell size produce undesirable debris during fabrication, maintenance, and utilization [29]. The way to alleviate this problem is to decrease the cell size. Microcellular foams are macroporous materials with cell size in the range of 0.1 to 100 μm. The processing of microcellular polymer and ceramic foams are well reported in the literature [30], [31], [32], [33]. A few methods are also reported for the preparation of microcellular carbon foams (MCF) [34], [35], [36], [37], [38], [39], [40]. In one of the methods, microcellular poly (acrylonitrile) foam obtained by a phase inversion process is pyrolyzed to obtain MCF [35]. Infiltration of phenol-formaldehyde resin into a sintered sodium chloride powder compact, followed by curing, pyrolysis, and sodium chloride removal is another method reported for the preparation of MCF [36], [37]. Lei et al. reported a process involving high-pressure foaming of an ethanol solution of phenol-formaldehyde resin in an autoclave followed by pyrolysis for the preparation of MCF with high compressive strength [38]. High-pressure foaming of pitch in an autoclave followed by pyrolysis and graphitization is also reported for the preparation of MCF [39], [40]. Shen et al. reported a hydrazine foaming method for the preparation of microcellular graphene foam with high electromagnetic interference (EMI) shielding from graphene oxide film [41]. Jiang et al. reported a preparation of microcellular graphene foam by chemical vapour deposition on microcellular Ni foam followed by leaching out the Ni foam substrate [42]. Li et al. reported the synthesis of ultrathin microcellular carbon foam by the pyrolysis of polyimide-graphene composite foam [43]. In this, the polyimide-graphene composite foam is obtained by casting graphene oxide dispersion in poly(amic acid) on a glass substrate followed by coagulation in water-alcohol mixture and thermal imidization at 350 °C.
Recently, we have reported a compression molding process for the preparation of carbon foams with a combination of macropores (70 to 414 μm), microcells (2 to 12 μm) and micropores (~ 1 nm) from sucrose and NaCl powder [44]. In this process, pastes of NaCl powder in molten sucrose are prepared by hot blending of sucrose-NaCl mixtures. The pastes are then set by caramelizing the sucrose in a steel mold by hot pressing at 160 °C. The compression molded bodies are subsequently pyrolyzed and washed with water to remove the NaCl template to produce the carbon foams. The macropores observed in the carbon foam are formed by the entrapped water vapour generated during caramelization and microcells are created by the removal of NaCl templates. The micropores are formed by the reaction of oxygen retained in the structure with the carbon at higher temperatures. The microstructure, foam density, compressive strength and thermal conductivity can be controlled by controlling the NaCl powder to sucrose weight ratios (WN/S) in the range of 0.7 to 1.2. The foams produced have a highly heterogeneous pore structure and the heterogeneity in pore structure decreases with an increase in WN/S. However, pastes of moldable consistency are not produced when the WN/S is increased beyond 1.2, as the molten sucrose is not sufficient to create an adequately thick layer around the NaCl particles to induce flow. This limits the achievable lower density and thermal conductivity of the carbon foams to 0.31 g/cm3 and 0.257 W/m·K, respectively. The versatility of a foam preparation process depends on its ability to produce foams with a wide range of density with a homogenous pore structure. In the present work, MCF of uniform pore structure (devoid of macropores) with density and thermal conductivity as low as 0.095 g/cm3 and 0.088 W− 1 m·K, respectively, is achieved by using glycerol as a plasticizing additive. Machining of carbon-NaCl composites using conventional machine tools followed by NaCl removal by leaching with water is used as a strategy to produce low-density carbon foams with desired contours.
Section snippets
Experimental
The analytical reagent grade sucrose, NaCl, glycerol, and methanol were procured from Merck India Ltd. Mumbai. The sucrose, NaCl powder, and glycerol were mixed by planetary ball milling (Fritsch, Germany) in methanol medium for 2 h at a speed of 200 RPM. Zirconia grinding balls of 10 mm diameter and zirconia jar of 500 ml capacity were used for ball milling. Methanol was selected as a solvent based on two reasons; (i) NaCl is only sparingly soluble (14.9 g/l) in methanol and (ii) glycerol is
Preparation of low-density microcellular carbon foams
Carbon-NaCl composites with high NaCl loading are required to achieve low-density microcellular carbon foams. Preparation of carbon-NaCl composites with high NaCl loading is not possible from sucrose-NaCl mixtures as the sucrose-NaCl mixtures of WN/S beyond 1.2 do not produce moldable paste upon hot blending at 185 °C [44]. At WN/S higher than 1.2, the molten sucrose is insufficient to produce an adequately thick layer on NaCl particle surface to induce plasticity. Thus, the incorporation of a
Conclusions
Moldable NaCl pastes in molten sucrose at NaCl to sucrose weight ratios (WN/S) in the range of 1.5 to 3 are prepared by using glycerol as a plasticizing additive. The setting of the molten sucrose-NaCl-glycerol pastes is due to the caramelization of sucrose as well as the slow evaporation of glycerol at 160 °C. High setting time (5 to 13 h) is due to slow caramelization of sucrose in glycerol medium. The carbon foams obtained by carbonization and NaCl template removal are devoid of macropores as
Acknowledgements
The authors are thankful to Dr. V.K. Dadhwal, Director, Dr. Kuruvilla Joseph, Dean, Student Activities, and Dr. Nirmala Rachel James, Head, Department of Chemistry, IIST for their encouragement. The authors also thank Dr. K. P. Surendran and Mr. K.S. Dijith of National Institute for Interdisciplinary Science and Technology for EMI shielding measurements.
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